Field of the Invention
The disclosure generally relates to the mooring systems for floating vessels. Specifically, the disclosure relates to the monitoring the condition of mooring systems of floating vessels, including offshore platforms.
Description of the Related Art
In the field of offshore oil and gas production, a moored vessel may be used as a floating drilling and/or production unit (FPU). Typically, an FPU is moored to the sea bed by means of multiple mooring lines attached to anchors, which may typically be pile anchors, suction anchors, or self-burying anchors of conventional type and which maintain the FPU in a desired location. Other vessels, such as offshore floating platforms including Spars, semi-submersible, and other platforms can also be similarly moored. The mooring line system can be a Taut or Semi-Taut or Taut Catenary or pure Catenary system.
An example of a typical mooring system for a Spar is illustrated in FIG. 1. FIG. 1 is schematic side view of a typical mooring system used for a floating vessel. FIG. 2 is a schematic top view of the typical mooring system with the floating vessel. The vessel 2 is held in a relative stable position while being allowed to float and move with the currents through a mooring system 4. A plurality of mooring lines 6 is stretched radially outwardly to a sea bed 8 and held at a mooring point 11 (such as an eye or other attachment means) on a pile or other anchoring device 10. Multiple mooring lines can extend from a given side and most often extend in multiple directions from the vessel as shown particularly in FIG. 2.
It is important that the mooring system is monitored in order to determine whether a line has been damaged, come loose from its anchoring point on the sea bed, or if the anchoring points have moved. If a single mooring line or its anchoring point is damaged in this way, the effect on the position of the vessel may not be particularly noticeable, but such damage must be recognised early in order that remedial action can be taken before further damage may allow the vessel to break free from its moorings and/or generate damage to the riser/export systems that are crucial to the production.
Historically, a typical mooring system failure is discovered during inspection by a diver or ROV. Some systems monitor the integrity of the mooring lines by a load cell or a compressive cell located either on the mooring line or on guiding/supporting equipment or at the tensioning system, by installing an inclinometer along the mooring line, or by installing a sonar deployed beneath the vessel. Such monitoring systems are expensive, complicated to install and maintain, and vulnerable to damage considering that they are installed underwater or close to water.
A further alternative for monitoring mooring systems is shown in US Publication No. 2010/0186652. The abstract states that the method of monitoring a vessel mooring system involves determining the geographical position of a locating point on the vessel remote from the mooring point and determining the heading of the vessel. The geographical position of the mooring point is then calculated from the determined position of the locating point and the vessel heading. The position of the mooring point is compared to at least one expected position of the mooring point, in order to provide an indication of failure of a mooring line or anchor. Because the geographical position varies with environmental conditions such as current flow and direction, wind speed and direction, and so forth, the geographical location by itself related to monitoring a mooring system would appear to have limited accuracy.
Vessels have their natural period in sway and surge (horizontal displacement) mainly linked to the stiffness of their mooring system. The mooring system causes the sway natural period to vary as a function of direction (that is, “heading”). FIG. 3 illustrates typical natural periods for various vessels that are deep water floating vessels, namely, a floating production storage and offloading (FPSO), deep draft floater (DDF, such as Spars), tension leg platforms (TLP), and semi-submersibles (Semi) in different modes of movement. The typical presence of risers, such as riser 12 in FIG. 2, results in an unsymmetrical inclination of the vessel to sway in the natural period with some directional dependence at a lower order.
A paper entitled “Mooring Design for Directional Spar Hull VIV” published at the Offshore Technology Conference in 2003 in Houston, Tex. as OTC 15243-MS (available at https://www.onepetro.org/conference-paper/OTC-15243-MS) discusses the effect of vortex induced vibrations (VIV) on a Spar mooring system when the Spar is fitted with helical strakes around the outer surface of the submerged hull and the effect on the natural period. The article discusses on page 2 one methodology of calculating Spar offsets due to imposed environmental current load and direction and drag loads due to hull VIV. The Spar sway natural period is determined based on the calculated offset and the mooring stiffness, and varies depending on whether the sway is in-line or out-of-line with the mooring lines. FIG. 4 shows variations of natural periods as a function of current speed and direction for a typical mooring system shown in FIG. 2. The mooring system can be a typical four direction, four mooring lines per direction (4×4) mooring configuration, illustrated in FIG. 2. The sway natural period (Tn) is normalized by the sway period at zero offset (Tno). The current speed varies from no current to a maximum design current. FIG. 5 illustrates variations in offsets with current velocity and heading. Variations due to the effects of the mooring restoring are shown in the directional behaviour of the offsets. In addition to the above paper not teaching monitoring mooring systems for degradation, the above paper aspects of natural periods but, like US Publication No. 2010/0186652, requires environmental factoring to achieve its results.
There remains then a need to provide more simplified yet predictable monitoring mooring system and method that can operate independently of factoring measurements for environmental conditions.